A massive fire roars through a mostly residential neighborhood in San Bruno, Calif., in 2010. A report released Friday by the Department of Transportation’s inspector general says the federal agency responsible for making sure states effectively oversee the safety of natural gas and other pipelines is falling down on the job.

WASHINGTON — The federal agency responsible for making sure states effectively oversee the safety of natural gas and other pipelines is failing to do its job, a government watchdog said in a report released Friday.

The federal effort is so riddled with weaknesses that it’s not possible to ensure states are enforcing pipeline safety, the report by the Transportation Department’s Office of Inspector General said. The federal Pipeline and Hazardous Materials Safety Administration, or PHMSA, isn’t ensuring key state inspectors are properly trained, inspections are being conducted frequently enough and inspections target the most risky pipelines, it said.

The findings come more than three years after a gas pipeline explosion and fire killed eight people, injured 58 and destroyed much of a subdivision in San Bruno, Calif., a suburb south of San Francisco. Accident investigators cited weak state and federal oversight.

The nation’s network of about 2.5 million miles of pipelines moves millions of gallons of hazardous liquids and 55 billion cubic feet of natural gas every day. Eighty-five percent of these pipelines are under state authority. The report doesn’t address the safety administration’s oversight of interstate pipelines like the proposed Keystone XL oil pipeline.

Cynthia Quarterman, head of the safety administration, defended her agency’s record, pointing to a two-thirds decline in the number of accidents and incidents involving gas distribution pipelines over the past 30 years.

There were 21 serious incidents on distribution pipelines in 2013 and 24 incidents in 2012, “which were the lowest number of serious incidents on record for the past 30 years,” she said in response to the inspector general’s findings included in the report.

Since 2010, when the San Bruno and four other major accidents occurred, she wrote, there have been no serious incidents to date on intrastate transmission lines, which are larger than distribution lines. But the agency also fully or partially agreed with seven recommendations made in the report.

Among the weaknesses cited in the report is that the safety administration is using an outdated formula to calculate the minimum number of inspectors states need. More inspectors may be needed to carry out new inspection methods and responsibilities since the formula was developed in the 1990s.

More than 20 percent of the nation’s total gas distribution pipelines are more than 50 years old or composed of material such as cast iron or bare steel that are more susceptible to failure than newer pipelines made with more resilient materials. However, the safety administration’s staffing formula also doesn’t take into account whether more personnel are needed to inspect these riskier pipelines, the report said.

The agency also hasn’t set minimum qualifications for state inspectors who lead inspection teams, the report said. In one state, for example, an inspector with less than one year’s experience was allowed to lead inspections, it said.

“Because it has not set minimum qualifications for state inspectors to lead standard inspections, PHMSA cannot be sure that state inspections cover all federal requirements and ensure pipeline operators maintain safety,” Assistant Inspector General Jeffrey Guzzetti said in the report.